It is well known that the reaction of lithium alkyls or aryls with methyl or ethylborate resulted in the formation of mixtures of products. EQU LiMe+B(OMe).sub.3 .fwdarw.LiMeB(OMe).sub.3 +LiMe.sub.2 B(OMe).sub.2 +LiMe.sub.3 BOMe+LiMe.sub.4 B
This presented a serious problem in the synthesis of pure boronic and borinic esters RB(OR').sub.2 and R.sub.2 BOR'.
I discovered that the problem could be overcome by a careful choice of the boronic or borinic ester used. Esters of secondary alcohols with boronic acid or borinic acids gave essentially quantitative yields of a single product. [H. C. Brown and T. E. Cole, Organometallics, 2, 1315-19 (1983); H. C. Brown, T. E. Cole and M. Srebnik, Organometallics, 4, 1788-92 (1985).]. For reasons of availability and cost, isopropyl alcohol is the preferred secondary alcohol. EQU LiR+B(Oi-Pr).sub.3 .fwdarw.LiRB(Oi-Pr).sub.3 EQU LiR+RB(Oi-Pr).sub.2 .fwdarw.LiR.sub.2 B(Oi-Pr).sub.2
In order to liberate the desired boronic ester or borinic ester from the complex, acids such as sulfuric acid or hydrogen chloride were used. EQU LiRB(Oi-Pr).sub.3 +HCl.fwdarw.LiCl+RB(Oi-Pr).sub.2 +i-PrOH EQU LiR.sub.2 B(Oi-Pr).sub.2 +HCl.fwdarw.LiCl+R.sub.2 BOi-Pr+i-PrOH
In practice, it was discovered that these by-product alcohols often form azeotropes with the desired boronic and borinic esters, RB(Oi-Pr).sub.2 and R.sub.2 BOi-Pr, azeotropes which greatly complicate separation of the alcohols and the esters. These alcohols interfere with the subsequent use of these boronic and borinic esters for the synthesis of lithium alkylsubstituted borohydrides [B. Singaram, T. E. Cole and H. C. Brown, Organometallics, 3, 774-777 (1984); B. Singaram, T. E. Cole and H. C. Brown, Organometallics, 3, 1522-25 (1984)]. EQU RB(Oi-Pr).sub.2 +LiAlH.sub.4 .fwdarw.LiRBH.sub.3 +(i-PrO).sub.2 AlH.dwnarw. EQU R'.sub.2 BOi-Pr+LiAlH.sub.3 OEt.fwdarw.LiR.sub.2 BH.sub.2 +i-PrOEtOalH.dwnarw.
The presence of free alcohol in the boronic and borinic esters results in the formation of difficultly separable mixtures.
The present invention overcomes the difficulties of the prior art processes and provides two improved procedures for liberating boronic, RB(OR').sub.2, and boronic, RR'BOR, esters from their "ate" complexes, free of alcohol.